Browsing by Author "Valente, A."
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- Biomanipulation of the Furnas Lake (Azores, Portugal)Publication . Azevedo, José M. N.; Valente, A.; Gonçalves, Vitor; Vieira, N.The Furnas Lake is located in the island of São Miguel, Azores. lt is a small lake (area, 1.92 km2;average depth, 6.9 m) with agricultura as the main activity in the catchment area (12.45 Km² ). Water temperaturas range from 10 °C to 24 °C. The water is turbid (Secchi disc depth is rarely > 1 m) and with a relatively high nutrient load (total N up to 2 mg.l ̄ ¹. total P up to 0.1 mg.l ̄ ¹ Turbid water and the frequent algae blooms caused by agricultura! runoff are considered a nuisance for tourism. The Regional Government has taken a series of measures of improve nutrient control, both upstream and in the water mass, but the problem has persisted. A two-year biomanipulation project started in March 2005 is aimed at reducing the cyprinid density (carp, Cyprinus carpio and roach, Rutilus rutilus) of the lake from 150 to about 20 kg.ha ̄ ¹ , and at monitoring the effects of this measure on water quality, macrophytes, zooplankton and the fish communities. The project work plan includes 1). the production of public information campaigns directed to ali lake users; 2). a survey of the initial state of the distribution and biomass of macrophytes, and species, size and biomass of zooplankton; 3). an intensiva removal of carp and roach during the reproductive season, employing gill and seine nets; and 4). to monitor the effects of biomanipulation on the water quality and biota.
- Coccolithophore species as indicators of surface oceanographic conditions in the vicinity of Azores islandsPublication . Silva, Alexandra A.; Brotas, Vanda; Valente, A.; Sá, C.; Diniz, Tania; Patarra, Rita F.; Álvaro, Nuno V.; Neto, Ana I.During summer 2008 and spring 2009, surface oceanographic surveys were carried out around three islands of the Azores archipelago (Terceira, São Miguel and Santa Maria) to assess the phytoplankton distribution and associated physico-chemical processes. The Azores archipelago is a major feature in the biogeochemical North Atlantic Subtropical Gyre (NAST) province although its influence on the productivity of the surrounding ocean is poorly known. Surface phytoplankton was studied by microscopy and HPLC (High Precision Liquid Chromatography). The mean values for biomass proxy Chlorophyll a (Chla) ranged from 0.04 to 0.55 μg Lˉ¹ (Chla maximum = 0.86 μg Lˉ¹) and coccolithophores were the most abundant group, followed by small flagellates, Cyanobacteria, diatoms and dinoflagellates being the least abundant group. The distribution of phytoplankton and coccolithophore species in particular presented seasonal differences and was consistent with the nearshore influence of warm subtropical waters from the south Azores current and colder subpolar waters from the north. The satellite-derived circulation patterns showed southward cold water intrusions off Terceira and northward warm water intrusions off Santa Maria. The warmer waters signal was confirmed by the subtropical coccolithophore assemblage, being Discosphaera tubifera a constant presence under these conditions. The regions of enhanced biomass, either resulting from northern cooler waters or from island induced processe, were characterized by the presence of Emiliania huxleyi. Diatoms and dinoflagellates indicated coastal and regional processe of nutrient enrichment and areas of physical stability, respectively.
- Effects of fish removal in the Furnas Lake, AzoresPublication . Bio, Ana; Couto, A.; Costa, R.; Prestes, Afonso L.; Vieira, N.; Valente, A.; Azevedo, José M. N.The Furnas Lake is a small volcanic, monomitic and increasingly eutrophised water body. Next to agricultural nutrient inputs, high densities of herbivorous fish are thought to contribute to high levels of turbidity in the lake, through zooplankton consumption and re-suspension of the nutrients accumulated in the sediment. According to the alternative state hypothesis a shift from turbid to clear water conditions is favoured by reduction of nutrient concentrations, increased light availability and reduction of planktivorous and benthos-feeding fish stock. To improve water quality in the Furnas Lake, a substantial part of the bottom-feeding fish population (62% of the estimated common carp population, Cyprinus carpio, and 5% of the estimated roach population, Rutilus rutilus) was removed. Effects of fish removal on turbidity and associated trophic state were analysed next to postmanipulation chlorophyll a concentration, zooplankton and macrophytes densities. Results suggest that fish removal was not enough to change lake conditions towards a lasting clear state dominated by macrophytes. Excessive nutrient load, in water and sediments, nutrient input from the lake basin and fish recruitment causing enhanced zooplankton grazing are appointed causes. Any further biomanipulation efforts should be associated to nutrient reduction; and continued monitoring of water quality, fish stock, macrophytes and zooplankton is needed.